Historical review shows that the treatment of popliteal aneurysm has developed by trial and error and there is disagreement about the proper management of the symptomless patient. In 1981 a policy of conservative management for asymptomatic popliteal aneurysm was adopted in this unit. Since that time we have also managed nine patients with thrombosed popliteal aneurysms by arteriography and low-dose intra-arterial streptokinase. Six patients treated within 72 h of occlusion achieved significant (70-100 per cent) lysis, but streptokinase was ineffective in those treated 10 or more days after the thrombosis. Of the six patients with significant lysis, three were treated by elective reconstruction and two by anticoagulation. One patient who had significant lysis died. Vascular patency of all five successfully treated limbs was maintained and no limb loss occurred in those who presented late and failed to achieve significant lysis. These results reinforce the view that thrombolysis is the treatment of choice for thrombosed popliteal aneurysms. The low complication rate for asymptomatic popliteal aneurysms and the advent of safe, effective thrombolysis indicate that operation for symptomless popliteal aneurysm is no longer required.